As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
As kinds of applications and products, to which the secondary battery is applicable, are increased, kinds of batteries are also increased such that the batteries can provide outputs and capacities corresponding to the various applications and products. For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, and laptop computers, use one or several small-sized, light secondary batteries (unit cells) for each device according to the reduction in size and weight of the corresponding products.
Based on the characteristics of external and internal structures thereof, the secondary battery may be generally classified as a cylindrical battery, a prismatic battery, or a pouch-shaped battery. Based on the construction of an electrode assembly, having a cathode/separator/anode structure, constituting the secondary battery, the secondary battery may be constructed in a jelly-roll (winding) type structure or a stacking type structure.
The jelly-roll type electrode assembly, which is generally used, is manufactured by coating a metal foil to be used as a current collector with an electrode active material, drying and pressing the coated metal foil, cutting the dried and pressed metal foil into the form of a band having a predetermined width and length, isolating an anode and a cathode from each other using a separator, and winding the anode/separator/cathode structure in a spiral shape. The jelly-roll type electrode assembly is generally suitable for cylindrical batteries. According to circumstances, the jelly-roll type electrode assembly may be compressed into a plate-shaped structure such that the jelly-roll type electrode assembly can be applied to prismatic batteries and pouch-shaped batteries.
FIG. 1 is a vertical sectional view illustrating the structure of a conventional cylindrical secondary battery.
Referring to FIG. 1, the cylindrical secondary battery 10 generally includes a cylindrical container 20, a jelly-roll type electrode assembly 30 mounted in the container 20, a cap assembly 40 coupled to the upper end of the container 20, and a crimping region 50 at which the cap assembly 40 is mounted.
The electrode assembly 30 is constructed in a structure in which a cathode 31 and an anode 32 are wound in a jelly-roll shape while a separator 33 is interposed between the cathode 31 and the anode 32. To the cathode 31 is attached a cathode tab 34, which is connected to the cap assembly 40. To the anode 32 is attached an anode tab (not shown), which is connected to the lower end of the container 20.
The cap assembly 40 includes a top cap 41 constituting a cathode terminal, a positive temperature coefficient (PTC) element 42 for intercepting electric current through the great increase of battery resistance when the interior temperature of the battery is increased, a bent safety member 43 for intercepting electric current and/or discharge gas when the interior pressure of the battery is increased, an insulating member 44 for electrically isolating the bent safety member 43 from a cap plate 45 excluding a specific portion, and the cap plate 45 connected to the cathode tab 34, which is attached to the cathode 31. The cap assembly 40 is constructed in a structure in which the top cap 41, the PTC element 42, the bent safety member 43, the insulating member 44, and the cap plate 45 are sequentially stacked.
The crimping region 50 is formed at the upper end of the container 20 such that the cap assembly 40 can be mounted to the open upper end of the container 20. More specifically, the crimping region 50 is formed by beading the upper end of the container 20, such that a depression 21 is formed at the inside of the container 20, mounting a gasket 60, sequentially inserting the outer circumferential parts of the cap plate 45, the insulating member 44, the bent safety member 43, and the top cap 41, and bending the upper end of the container 20. As a result, the crimping region 50 is formed in the shape to surround the gasket 60 located at the inside of the crimping region 50. The cap assembly 40 is mounted at the crimping region 50 by crimping and pressing.
However, it has been proven that, when external impacts are applied to the secondary battery with the above-stated construction, the sealability of the secondary battery is decreased, the resistance at the electrical connection regions of the secondary battery is changeable, and the safety of the secondary battery is lowered, whereby it is difficult for the secondary battery to exhibit desired battery performance.
For this reason, the inventors of the present invention proposed a secondary battery having an improved structure, which is disclosed in Korean Patent Application No. 2006-22950 (see FIG. 2).
Referring to FIG. 2, the cylindrical secondary battery 100 is manufactured by inserting an electrode assembly 110 into a container 200, injecting an electrolyte into the container 200, and mounting a cap assembly 300 to the open upper end of the container 200. The process for manufacturing the cylindrical secondary battery 100 is generally identical to the process for manufacturing the conventional cylindrical secondary battery. However, the structure of the cylindrical secondary battery 100 is different from that of the conventional cylindrical secondary battery. The difference will be described below in detail.
The cap assembly 300 is mounted to the open upper end of the container 200 by a crimping region 500 constructed in a structure in which a top cap 310 and a bent safety member 320 for lowering the interior pressure of the battery are in tight contact with each other inside a gasket 400 mounted to an upper beading part 210 of the container 200 for maintaining airtightness. The top cap 310 is formed such that the central part of the top cap 310 protrudes upward, and therefore, the top cap 310 serves as a cathode terminal, to which an external circuit is connected. The top cap 310 is provided along the circumference of the protruding part thereof with a plurality of through-holes 312, through which pressurized gas is discharged out of the container 200.
The bent safety member 320 is a thin-film structure through which electric current flows. The central part of the bent safety member 320 is depressed to form a depressed central part 322, and two notches 324 and 326 having different depths are formed at upper and lower bent regions of the central part 322, respectively. Below the bent safety member 320 is mounted a current intercepting member 600 for discharging gas out of the battery and, at the same time, intercepting electric current. The end 328 of the bent safety member 320 surrounds the outer circumference surface 314 of the top cap 310, and an annular protrusion 316 is formed at the lower end surface of the top cap 310.
The cylindrical secondary battery 100 with the above-stated construction solves the sealability-related problem and the resistance change problem at the electrical connection regions. However, the experiments carried out by the inventors of the present invention revealed that, when strong external impacts are applied to the secondary battery or the internal pressure of the secondary battery is abruptly increased, the crimping region constructed in the structure of FIG. 1 as well as the crimping region constructed in the structure of FIG. 2 is easily deformed, with the result that the contact surfaces between the top cap, the bent safety member, and the gasket are separated from each other, whereby the sealability of the secondary battery is lowered.
FIG. 3 is a partially enlarged view illustrating the crimping region of the cylindrical secondary battery. For convenience of description, only the section of the container forming the crimping region is shown.
Referring to FIG. 3, the crimping region 500 is constructed in a structure in which the end of the crimping region 500 is bent such that the cap assembly 300 (see FIG. 2) is stably mounted to the open upper end of the container 200 (see FIG. 2) while the gasket 400 (see FIG. 2) is disposed between the cap assembly 300 and the open upper end of the container 200. A sidewall 520 of the crimping region 500 is vertically formed in the same manner as the side of the battery, and an bent front end 510 of the crimping region 500 extends inward while the bent front end 510 of the crimping region 500 is inclined in a predetermined angle such that the bent front end 510 of the crimping region 500 presses the gasket 400 (see FIG. 2).
In this structure, however, when a strong external force is applied to the side of the battery (in the direction indicated by a horizontal arrow), for example, the sidewall 520 of the crimping region 500 is bent or pressed inward, with the result that the bent front end 510 of the crimping region 500 moves upward (see line a). Also, when a strong external force is applied to the corner of the battery (in the direction indicated by an approximately 45 degree-declining arrow), for example, the sidewall 520 of the crimping region 500 is protruded outward, with the result that the bent front end 510 of the crimping region 500 spreads (see line b). These deformations may act as a factor lowering the sealing force of the gasket by the crimping region 500.
On the other hand, when the battery is exposed to a high-temperature environment or a local short circuit occurs in the battery due to external impact applied to the battery, the decomposition reaction of an electrolyte occurs at the cathode interface, with the result that a large amount of gas is generated. The generated gas increases the internal pressure of the battery. When the internal pressure of the battery exceeds a predetermined pressure level, the bent safety member 320 (see FIG. 2) is operated to discharge the high-pressure gas to the outside. However, the crimping region is deformed due to the high-pressure gas, before the operation of the bent safety member 320, the sealability of the gasket is lowered, with the result that, the electrolyte is discharged out of the battery together with the high-pressure gas, and therefore, the safety of the battery is greatly deteriorated.
Specifically, when the crimping region is deformed, the contact surfaces between the top cap, the bent safety member, and the gasket are separated from each other, or the bent front end of the crimping region cannot strongly press the gasket, and therefore, the sealability of the gasket is partially released. As a result, the contact surfaces between the bent safety member and the top cap are instantaneously separated from each other. Consequently, the electrolyte leaks out of the battery through the gap, and therefore, the battery may catch fire or explode. As a result, the safety of the battery is greatly lowered.
Therefore, there is a high necessity for a cylindrical secondary battery that is capable of maintaining the sealability from external impacts or internal pressure.